The mobile radio systems first in common use had analog transmission of information between base stations and mobile stations, i.e. analog information was transmitted between base and mobile stations by using analogouslymodulated radio signals on analog radio channels. By frequency multiplexing technique analog mobile radio systems may have plural radio channels. The Nordic mobile telephone system, NMT, is an example of such an analog mobile radio system having many radio channels.
Recently there has been suggested mobile radio systems with digital transmission of information between base stations and mobile stations, i.e. digital or digitized information is transmitted between base and mobile stations using digitally modulated radio signals on digital radio channels. Of course more radio channels may be created by frequency multiplex in digital mobile radio systems also. In order to get even more channels in digital mobile radio systems it has been proposed that digital radio channels shall share a radio frequency in time multiplex. The pan european mobile radio system, GSM, may be mentioned as an example of such a digital mobile radio system.
The digital technique means substantial advantages in mobile radio systems. Accordingly there is an interest in introducing the digital technique. On the other hand there are in certain areas existing analog mobile radio systems using the frequency band available for a digital system. When huge investments have been made in existing analog systems it is not always reasonable to immediately replace an existing analog system with a digital system. It has therefore been suggested to successively replace analog channels in analog systems having many analog channels in frequency multiplex by a number of digital channels in time multiplex. For the United States it has been suggested that each analog channel shall be replaced by three digital channels in time multiplex. During a long transitory period of time such a mobile radio system would then comprise both analog radio channels for analog transmission of information by using analog-modulated radio signals and digital radio channels for digital transmission of information by using digitally modulated radio signals. Mobile stations which during the transitory time wish to be able to use the full capacity of such a system must then be able to use both analog radio channels and digital radio channels.
In digital mobile radio systems problems occur with time dispersion on the digital radio channels due to reflections and multipath propagation unless particular measures are taken. The time dispersion changes from one place to the other due to the radio propagation properties. (Time dispersion problems have long been known in the fields of radio and television. For example, "ghosting" on a television screen is a result of time dispersion problems associated with receiving a broadcast signal and, after a time delay, receiving one or more reflections of that signal. In digital mobile radio systems, a plurality of "ghosts" may occur: some weak and some quite strong. Equalizers are used to interpret the broadcast signal and signals resulting from reflections or multipath propagation to reconstruct the transmitted signal. Equalizers must interpret these signals where the transmitted signal and its "ghosts" are of substantially the same signal strength, are of differing signal strengths, have a low time dispersion or have a severe time dispersion. Of course, high signal strength may be associated with low or severe time dispersion as may low signal strength.) In order to obtain a reliable transmission of information in spite of time dispersion it is well known to have equalizers at the receiving side in a mobile radio system. Depending upon design such an equalizer can handle shorter or longer time dispersion. One way to describe the ability of an equalizer to cope with time dispersion is to state the maximum time dispersion in microseconds the equalizer is designed for. Another way is to state how many symbol time spans the equalizer can cope with. The complexity, cost and power consumption of an equalizer increases progressively with the maximum time dispersion in microseconds and the number of symbol duration times. Accordingly there is an interest in designing a mobile radio system such that the requirements on the equalizers do not become too great and in particular not become greater than necessary. For the above mentioned pan european system GSM it has been decided that the equalizers shall be designed to cope with a maximum time dispersion of 16 microseconds, which corresponds to four symbol time intervals. In the above mentioned mobile radio system for the United States having both digital and analog radio channels the system should probably need to cope with a maximum time dispersion of about 60 microseconds on the digital radio channels, which is more than the duration of one symbol but less than the duration of two symbols.
The analog transmission of information on the analog radio channels of existing analog mobile radio systems, e.g. the above mentioned NMT or of the type existing in the USA, is normally substantially less sensitive than digital systems to reflections and multipath propagation causing dispersion on the digital radio channels.